Manhattan Project Timeline
Timeline Description: The Manhattan Project is the name given to the U.S. atomic weapons program during World War II. This program produced the first atomic weapons, eventually leading to the bombings of Hiroshima and Nagasaki in August 1945.

Date Event
August 2, 1939 Einstein Signed the Einstein-Szilard Letter

On August 2, 1939, Albert Einstein signed the Einstein-Szilard Letter. This letter encouraged Roosevelt to actively begin work on an atomic weapon to potentially take action against the growing power of Nazi Germany.
September 3, 1939 World War II Began in Europe

World War II began with the Nazi invasion of Poland in 1939. Britain provided an ultimatum, and when the time ticked down on that ultimatum, war began.
October 11, 1939 Einstein-Szilard Letter Delivered to Roosevelt

The Einstein-Szilard letter was delivered to Roosevelt by economist Albert Sachs. Roosevelt immediately established a Uranium Committee to begin research.
October 21, 1939 First Meeting of Uranium Committee

Lyman Briggs handled the first meeting of the Uranium Committee. The Committee was given a $6,000 budget to begin their work.
March 2, 1940 Neil Bohr's Hypothesis Verified

Neil Bohr's hypothesis that Uranium 235 is responsible for fission is verified at Columbia University.
June 12, 1940 National Research Defense Committee Created

In June 1940, the National Research Defense Committee was created, absorbing the Uranium Research Committee. The budget was increased to $40,000.
February 25, 1941 Discovery of Plutonium

Plutonium was conclusively discovered on February 25, 1941 by Glenn Seaborg and Arthur Wahl.
June 28, 1941 Office of Scientific Research and Development Created

The Office of Scientific Research and Development was created on June 28, 1941. The OSRD absorbed the National Research Defense Committee.
December 7, 1941 Attack on Pearl Harbor

On December 7, 1941, the Japanese attacked the U.S. naval base at Pearl Harbor, Hawaii. This forced the immediate entry of the United States into World War II.
January 19, 1942 Roosevelt Authorized Atomic Bomb Project

In January 1942, Franklin D. Roosevelt authorized the Manhattan Project to develop atomic weapons to use against the enemies of the United States.
September 29, 1942 Acquisition of Site X

The U.S. government acquired land in Oak Ridge, Tennessee as the first research site for the Manhattan Project. This was called Site X.
November 16, 1942 Los Alamos Declared Site Y

Los Alamos, New Mexico was chosen as Site Y for the development of an atomic bomb.
April 1, 1943 Los Alamos Laboratory Established

The research laboratory at Los Alamos was established in April 1943. This research laboratory would be fully responsible for the development of the atomic bomb.
January 11, 1944 Special Group Appointed to Study Implosion at Los Alamos

On January 11, 1944, a special group was appointed to study the possibilities of implosion rather than explosion.
July 4, 1944 Thin Man Abandoned, Fat Boy Prioritized

The explosion-based Thin Man project was fully abandoned in July 1944. Fat Boy, an implosion based weapon, and Little Boy, a gun-based weapon, remained in development.
May 7, 1945 Surrender of Nazi Germany

Nazi Germany surrendered on May 7, 1945, or Victory Europe day. The war in the Pacific raged on.
July 16, 1945 Test of the Gadget

On July 16, 1945, the U.S. conducted the first test of an atomic weapon, called the Gadget.
July 25, 1945 Atomic Weapon Use Authorized

The United States authorized the use of atomic weapons against key targets in Japan, including Hiroshima and Nagasaki on July 25. The following day, the U.S. provided Japan with a final warning about devastating damage.
August 6, 1945 Little Boy Deployed on Hiroshima

On August 6, 1945, Little Boy was deployed by the B-29 bomber Enola Gray. Little Boy was a gun-type Uranium 235 based weapon.
August 6, 1945 Fat Man Deployed on Nagasaki

As the Enola Gray bombed Hiroshima, another B-29, the Bockscar, dropped a plutonium based implosion bomb on Nagasaki.
August 12, 1945 Surrender of Japan

Japan offered its unconditional surrender on August 12, 1945, commonly called Victory Japan day in the United States.
January 1, 1947 Atomic Energy Act of 1946

The Manhattan Project was officially dismantled on January 1, 1947 in response to the Atomic Energy Act of 1946.